Laser anneal method of a semiconductor layer

ABSTRACT

For obtaining p-Si by irradiating a laser beam to an a-Si layer to polycrystallize, an energy level in a region to be irradiated by the laser beam is set such that a level at the rear area of the region along a scan direction of the laser beam is lower than that at the front area or the center area of the region. The energy level at the front area or the center area of the region is set such that it is substantially equal to or more than the upper limit energy level which maximizes a grain size of the p-Si obtained. Since an energy profile is set as described above, when the laser beam is scanned on the a-Si layer, an irradiated energy of the laser on the region is gradually lowered from the upper limit as the laser beam passes through, which allows the semiconductor layer to be annealed within an optimal energy level during the latter half of the annealing process.

BACKGROUND OP THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for manufacturing asemiconductor device, particularly to a method for manufacturing an LCD(liquid crystal display) such as a driver circuit integrated type LCD inwhich TFTs (thin film transistors) comprising a polycrystalsemiconductor layer are formed in a display area and a driver area.

[0003] 2. Description of the Related Art

[0004] In recent years, LCDs have been regularly employed in OA and AVapparatuses because of advantages resulting from their small size andthickness and their low power consumption. Active matrix type displays,in which each pixel is equipped with a TFT as a switching device forcontrolling the rewrite timing of image data, are especially able todisplay moving pictures with high resolution on a large screen, and aretherefore used for displays in various televisions, personal computers,and the like.

[0005] A TFT is an FET (field effect transistor) obtained by forming asemiconductor layer together with a metal layer in a predetermined shapeon an insulating substrate. In an active matrix type LCD, each TFT isconnected to an electrode of each pixel capacitor formed between a pairof substrates, for driving liquid crystal.

[0006] In particular, developments have been made to LCDs usingpolycrystal silicon (p-Si) as a semiconductor layer in place ofamorphous silicon (a-Si) which has previously been common, and annealingwith use of a laser beam has been put to use for formation or growth ofcrystal grains of p-Si. In general, p-Si has a higher mobility than a-Siso that using p-Si to form a TFT can downsize TFTs, allowing a highaperture ratio and a high resolution to be realized. In addition, sinceTFTs can be constructed in a gate self-alignment structure, fine TFTelements can achieve higher speed operation by reductions in parasiticcapacity. By using these TFTs to form an electric complementaryconnection structure between an n-ch TFT and a p-ch TFT, i.e., a CMOS, ahigher speed driver circuit can be constructed. Therefore, a drivercircuit section can be formed to be integrated with a display pixelsection on one substrate, allowing manufacturing costs to be reduced andrealizing a small size LCD module.

[0007] Known methods of forming a p-Si layer on an insulating substrateinclude a crystallization method under a high temperature, by annealinga-Si formed under a low temperature, a solid phase crystallizationmethod under a high temperature, and the like. In all known methods,some treatment must be carried out under a high temperature of 900° C.or more. Therefore, it is not possible to use a low cost non-alkalineglass substrate as an insulating substrate in view of heat resistanceand, as quartz glass substrate is required, a higher manufacturing costresults. In contrast, developments have been made to a method whichallows use of a non-alkaline glass substrate as an insulating substrateby performing silicon polycrystallization processing at a relatively lowsubstrate temperature of 600° C. or less, through use of laserannealing. Such processes, in which the processing temperature is 600°C. or less throughout all TFT manufacturing steps are called“low-temperature processes”, and are essential for mass-production oflow cost LCDs.

[0008]FIG. 1 shows a state of a substrate to be processed by excimerlaser annealing (hereinafter referred to as “ELA”). A substrate 1 to beprocessed is a popular non-alkaline glass substrate. An a-Si layer isformed on the surface of the substrate 1. An active matrix substrate 5is a substrate for constructing an LCD comprising a display area 2 wheredisplay pixels are arranged in matrix, and a gate driver 3 and a draindriver 4 provided surrounding the display area 2. The substrate 1 is amother glass substrate including a plurality of active matrix substrates5. In the display area 2, pixel electrodes, each being an electrode of apixel capacitor for driving liquid crystal, are formed and arranged inmatrix, and are respectively connected with TFTs formed. The gate driver3 is mainly constructed by a shift register, and the drain driver 4 ismainly constructed by a shift register and a sampling circuit. Thesedrivers are formed by a TFT array such as a CMOS or the like.

[0009] Each of TFTs is formed such that, as shown in FIG. 2, a p-Silayer obtained by crystallization of an a-Si layer by use of the ELAmethod is used as an active layer. In the area where a p-Si layer 11etched into an island-like shape is formed, a non-doped channel regionCH, light-doped regions LD, and heavy-doped source and drain regions S,D are arranged. On the channel region CH, a gate electrode 13 isarranged with a gate insulating film interposed between the channelregion and the gate electrode 13. A source electrode and a drainelectrode are connected to the source and the drain regionsrespectively. In the driver circuit areas, a TFT is connected to form aCMOS or the like. In a display area, a signal line and a pixel electrodeare connected to the respective drain electrode and the sourceelectrode.

[0010] As shown in FIG. 1, in a conventional laser annealing method, aline beam is irradiated on a substrate 1 such that the contour of edgelines C of a band-like irradiated region of a line beam irradiated onthe substrate 1 is shifted by a predetermined overlap amount. Scanningis carried out as indicated by the arrow in the drawing, and the entiresubstrate is subjected to annealing. However, after scanning is thusperformed with a line beam, there remains a defective crystallizationregion in which sufficient crystallization was not attained and grainswith a smaller grain size, as indicated by reference R in the figure,remain in p-Si formed. This region is formed in a fine liner shape alongthe longitudinal direction of the irradiated region, and appears in astriped pattern. Since this defective crystallization region R has a lowmobility and a high resistance, the characteristics of TFTs formed inthis region are degraded. If the characteristics of TFTs are thusdegraded, pixel capacitors are not sufficiently charged in the displayarea so that the contrast ratio is lowered, or erroneous operation iscaused in the driver circuit area, thus disadvantageously influencingdisplay.

[0011] It is estimated that a defective crystallization region asdescribed above is caused because of unevenness in energy of anirradiated laser beam. Laser annealing strongly depends on the energy ofthe irradiated laser beam. In general, the grain size of crystal tendsto increase as the irradiation energy increases. However, when theenergy level increases to a certain level Eu or more, the grain sizerapidly decreases to the microcrystal level. Hence, it is demanded thatthe energy level of a laser beam to be irradiated onto an a-Si layershould be as large as possible within a range of Eu to Ed which is lowerthan an upper limit level Eu such that the energy level does not exceedthe upper limit Eu, in order to enlarge the grain size as much aspossible thereby to achieve TFTs having excellent characteristics.

[0012]FIG. 3 shows an energy distribution of an irradiation beam withrespect to positions in a line beam. An optical system for generating aline beam is provided with a line width adjust slit and a line lengthadjust slit, to form a line beam of a band-like or rectangular shape.Thus, since the line width A of the line beam is defined by the linewidth adjust slit, the characteristic curve of the irradiation lightintensity distribution has substantially sharp edges and a substantiallyflat energy distribution peak portion Eo, as shown in FIG. 3. However,in legions X and B in FIG. 3, the energy level is extremely high or lowand is thus greatly differs from the level at the flat portion.

[0013] In an optical system comprising a plurality of lenses, light isdiffracted or interfered due to slight concave and convex portionsexisting in the lens surfaces or foreign material contamination or thelike adhering thereto. The light thus diffracted or interfered isconverged in the line width direction A and is expanded in the linelength direction, so that nonuniformity of energy of the laser beamirradiated toward the substrate 1 from the optical system is increased.Even slight amounts of foreign material or the like present in a cleanroom, may cause nonuniformity in light intensity. Therefore,nonuniformity of the output energy of a line beam cannot be completelyeliminated at present, and it is unavoidable that the energy level of aline beam to be irradiated partially exceeds the upper limit whichallows an appropriate grain size.

[0014] As a result of this, a line beam whose energy level is uneven isintermittently irradiated as shown in FIG. 3, and a laser beam whichpartially exceeds the upper limit Eu of the energy level is irradiatedwithin a unit irradiated region having edge lines C as shown in FIG. 1.It is therefore considered that a much finer linear defectivecrystallization region R is caused within the edge lines C.

SUMMARY OF THE INVENTION

[0015] The present invention has an object of providing a method ofpreventing nonuniformity in semiconductor layer characteristics and ofobtaining a semiconductor layer with excellent characteristics by meansof laser anneal processing.

[0016] To achieve the above object, the present invention provides alaser anneal method for improving quality of a semiconductor layerformed on a substrate by irradiating a laser beam or for obtaining apolycrystal semiconductor layer from an amorphous semiconductor layer,wherein an energy level of the laser beam in a region to be irradiatedis set such that a level towards the rear of the region along which thelaser beam scans is lower than that at the front area or the center areaof the region.

[0017] Thus, after a high energy part of the laser beam at a relativelyforward part with respect to the scan direction passes through thesemiconductor layer, a lower energy part passes through thesemiconductor layer sequentially. Therefore, after large crystal grainsare initially formed by the high energy part, remaining defectivecrystallization regions are crystallized by the relatively low energypart while the large crystal grains are maintained, thereby improvingcrystallinity for the entire region.

[0018] In another aspect, the present invention provides a laser annealmethod of a semiconductor layer for obtaining a polycrystalsemiconductor layer by irradiation of an amorphous semiconductor layerformed on a substrate with a laser beam, wherein an energy level at aregion to be irradiated by the laser beam is set such that the peaklevel in the rear area of the region along a scan direction of the laserbeam is lower than the upper limit the energy level that maximizes thesemiconductor layer grain size.

[0019] Thus, the irradiated energy does not exceeds the upper limitenergy level which maximizes the grain size at the rear area of theirradiated laser beam along the scan direction passing over theamorphous semiconductor layer sequentially, therefore it is possible toprevent the semiconductor layer from being changed into an amorphousstate, and to carry out crystallization with defective crystallizationregions. Accordingly, a polycrystal semiconductor layer having an almostuniform grain size over the entire region of the semiconductor layer canbe formed.

[0020] In a further aspect, in addition to the above-describedcondition, the peak level of the laser beam at the front area or thecenter area of the region is substantially equal to or more than theupper limit energy level which maximizes the grain size of thesemiconductor layer.

[0021] In yet another aspect of the present invention, the upper limitenergy level which maximizes the grain size of the semiconductor layeris corresponds to the lower limit energy level over which thepolycrystal semiconductor layer is changed into an amorphous state.

[0022] Thus, the level of the irradiated laser beam passing over thesemiconductor layer exceeds the upper limit energy level in some casesbecause the energy level at the front area is relatively high, allowingthe semiconductor layer to be microcrystallized by such a laser beam.However, the energy level irradiated to the semiconductor layergradually decreases as the laser beam scanning proceeds from the frontarea to the rear area because the energy level is set such that a levelat the rear area is lower than that at the front, which allows theenergy level irradiated to the semiconductor layer to not exceed theupper limit energy level and to yet be sufficiently high and optimal.The latter half of the laser annealing process for a predeterminedregion of the semiconductor layer is performed in an energy range inwhich a sufficiently large grain size can be formed by employing such anenergy profile of the laser beam, because an energy range which isoptimal to maximize the grain size is just below the upper limit.Therefore, the laser anneal processing is performed under the bestconditions. As a result, a polycrystal semiconductor layer with a largegrain size and excellent uniformity is formed.

[0023] According to another aspect of the present invention, the laserbeam irradiated on the amorphous semiconductor layer is obtained byshaping a laser beam generated from a laser oscillation source by anoptical system including a plurality of lenses, such that the region tobe irradiated has a predetermined shape, and the energy level, energydistribution, or their combination in the region to be irradiated by thelaser beam is controlled by adjusting a distance between the amorphoussemiconductor layer formed on the substrate and the focal point of thelaser beam formed by the optical system.

[0024] By thus controlling the distance between the substrate on whichthe amorphous semiconductor layer is formed and the focal point of thelaser beam formed by the optical system, the energy profile of the laserbeam is easily modified in a desired shape, thereby allowing a betterlaser anneal processing to be performed by a simple control.

[0025] According to yet another aspect of the present invention, thescan direction of the laser beam is set such that an energy level at therear area of the region along the scan direction is lower than the upperlimit energy level, which allows the laser profile along the scandirection to be more simple and suitable.

[0026] In another aspect, the present invention is a transistor devicein which a polycrystal semiconductor layer is formed by subjecting anamorphous semiconductor layer formed on a substrate to laser annealprocessing, wherein an energy level in a region to be irradiated by alaser beam of the amorphous semiconductor layer is set such that thelevel at the rear area of the region along a scan direction of the laserbeam is lower than the upper limit energy level which maximizes a grainsize of the semiconductor layer, and the amorphous semiconductor layeris annealed by the laser beam and the polycrystal semiconductor layerobtained is used as an active layer of the transistor device.

[0027] According to yet another aspect of the present invention of thetransistor device; the transistor device is a thin film transistor, anda channel layer of this thin film transistor is formed in thepolycrystal semiconductor layer obtained by the laser anneal processing.

[0028] In a further aspect of the present invention, the transistordevice is a thin film transistor, a channel layer of this thin filmtransistor is formed in the polycrystal semiconductor layer obtained bythe laser anneal processing, and the thin film transistor is used as aswitching device formed in a display area of a substrate forming aliquid crystal display and as a switching device of a driver circuitformed surrounding the display area of the substrate through a processsubstantially equal to a process of forming the switching device of thedisplay region.

[0029] By thus controlling the laser energy to form a polycrystalsemiconductor layer and by using the layer as an active layer of atransistor device, for example a thin film transistor, it is possible toobtain a transistor device with a high speed and excellentcharacteristics. Further, since a polycrystal semiconductor layer havingexcellent characteristics is obtained, it is possible to form atransistor device having a gate-self-align structure. If this kind oftransistor is used as a switching device of a display area and as aswitching device of a driver circuit area for driving the switchingdevice of the display area in the liquid crystal display or the like, itis possible to form a display with excellent display quality.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a view explaining a conventional positional relationshipbetween a substrate to be processed and a region to be irradiated by aline beam.

[0031]FIG. 2 is a view showing a planar construction of a TFT formed bya conventional laser anneal.

[0032]FIG. 3 is a graph explaining an energy profile of an irradiationlaser beam.

[0033]FIG. 4 is a view explaining a positional relationship between asubstrate to be processed and a region to be irradiated by a line beamaccording to an embodiment of the present invention.

[0034]FIGS. 5, 6, 7 and 8 are graphs explaining an energy profile of aline beam according to an embodiment of the present invention.

[0035]FIG. 9 is a graph showing a relation between laser beamirradiation time (the number of pulse shots) and temperature of a filmto be processed in the ELA according to an embodiment of the presentinvention.

[0036]FIG. 10 is a graph showing a relation between laser beamirradiation time (the number of pulse shots) and a grain size of a filmto be processed in the ELA according to an embodiment of the presentinvention.

[0037]FIG. 11 is a view showing a correlation between beam profile andstate of a film to be processed in an ELA according to an embodiment ofthe present invention.

[0038]FIG. 12 is a view showing a correlation between a beam profile, astate of a film, a position in the film, and a grain size at a timing inthe ELA according to an embodiment of the present invention.

[0039]FIGS. 13A, 13B and 13C are views showing microphotographs showingthe view of the state of a film shown in the middle section of FIG. 12.

[0040]FIG. 14 is a view showing a correlation between beam profile andstate of a film to be processed in an ELA according to an embodiment ofthe present invention.

[0041]FIG. 15 is a view showing a correlation between a beam profile, astate of a film, a position in the film, and a grain size at a timing inthe ELA according to an embodiment of the present invention.

[0042]FIGS. 16A, 16B and 16C are views showing color microphotographsshowing the state of a film shown in the middle section of FIG. 15.

[0043]FIG. 17 is a graph showing a relation between laser energy andgrain size in an ELA according to an embodiment of the presentinvention.

[0044]FIG. 18 is a schematic view of the structure of a laserirradiation apparatus used in an embodiment of the present invention.

[0045]FIGS. 19 and 20 are views explaining the structure of an opticalsystem of a laser irradiation apparatus shown in FIG. 18.

[0046]FIG. 21 is a graph showing a beam profile of a laser beam used inan ELA according to Example 1 of the present invention.

[0047]FIGS. 22A, 22B and 22C are views showing microphotographs of ap-Si layer when the scan direction of a laser beam shown in FIG. 21 isset in the left direction in FIG. 21.

[0048]FIGS. 23A, 23B and 23C are views showing microphotographs of ap-Si layer, when the scan direction of a laser beam shown in FIG. 21 isset in the right direction in FIG. 21.

[0049]FIG. 24 is a graph showing a beam profile of a laser beam used inan ELA according to Example 2 of the present invention.

[0050]FIGS. 25A, 25B and 25C are views showing microphotographs of ap-Si layer when the scan direction of a laser beam shown in FIG. 24 isset in the left direction in FIG. 24.

[0051]FIGS. 26A, 26B and 26C are views showing microphotographs of ap-Si layer when the scan direction of a laser beam shown in FIG. 24 isset in the right direction in FIG. 24.

[0052]FIG. 27 is a graph showing a beam profile of a laser beam used inan ELA according to Example 3 of the present invention.

[0053]FIGS. 28A, 28B and 28C are views showing microphotographs of ap-Si layer when the scan direction of a laser beam shown in FIG. 27 isset in the left direction in FIG. 27.

[0054]FIGS. 29A, 29B and 29C are views showing microphotographs of ap-Si layer when the scan direction of a laser beam shown in FIG. 27 isset in the right direction in FIG. 27.

[0055]FIG. 30 is a view showing a planar construction of a TFT formed bya laser anneal according to an embodiment of the present invention.

[0056]FIG. 31 is a view schematically showing a cross-section of thestructure of the TFT shown in FIG. 30.

[0057]FIG. 32 is a view schematically showing a cross-section of thestructure of a display area in a liquid crystal display apparatus usinga TFT according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058]FIG. 4 is a plain view for showing a state of a substrate to beprocessed by an ELA according to an embodiment of the present invention.The substrate 7 to be processed is a popular non-alkaline glasssubstrate, and an a-Si layer is formed on its surface. An active matrixsubstrate 25 forms one substrate of an LCD comprising a display area 22where display pixels are formed and arranged in matrix, and a gatedriver 23 and a drain driver 24 formed and arranged surrounding displayarea 22. The substrate 7 is a mother glass substrate including sixactive matrix substrates 5. In the display area 22, pixel electrodes,each being an electrode of a pixel capacitor for driving liquid crystal,are arranged in matrix and each TFT is formed to be connected with eachrespective pixel electrode. The gate driver 23 is mainly constructed bya shift register, and the drain driver 24 is mainly constructed by ashift register and a sampling circuit. These drivers are formed by a TFTarray such as a CMOS or the like, and each TFT is formed with use ofp-Si formed by the ELA method according to the present invention, as achannel and a source/drain layer.

[0059] In an embodiment of the present invention, ELA is performed byirradiating a line beam obtained from a laser irradiation apparatus, aswill be described below, onto a substrate 7 as a substrate to beprocessed, to scan the substrate with the beam. An excimer laser is apulse laser, and a line beam of the pulse laser is intermittentlyirradiated onto the substrate 7 to be processed such that the substrate7 is scanned in the direction indicated by the arrow in FIG. 4(corresponding to the longitudinal direction in the figure) with theline beam. Further, the line beam is controlled such that irradiatedregions of any successive pulse beams overlap each other by apredetermined amount (See edge lines C in the figure.) The entiresurface of an amorphous semiconductor layer on the substrate 7 isannealed by scanning the substrate 7 while thus sequentially shiftingthe position of the line beam to be irradiated.

[0060] A line beam generated by a laser irradiation apparatus, to bedescribed below, has an approximate line length of 80 to 300 mm and aline width of 0.1 to several mm.

[0061] Scanning is performed such that the line beam is moved asdescribed above in the line width direction on the substrate 7, and iscarried out for each active matrix substrates 25 arranged in a pluralityof columns (e.g., in two columns in FIG. 4) on the substrate 7 to beprocessed as a mother substrate. In this manner, annealing is performedonce on the entire surface of the substrate 7 to be processed.

[0062] An energy profile of a line beam, according to the presentembodiment, for obtaining polycrystal silicon with a preferable grainsize is next explained referring to FIGS. 5 to 8. The present embodimentis based on the fact that a final grain size of a film is influenced byan energy level at the latter half of the laser anneal processing, andemploys a line beam having an energy profile suitable for an annealingprocess in an energy range which maximizes the grain size.

[0063] FIGS. 5 to 8 are energy profiles devised from such a point ofview, and each of the figures shows a relation between a position of airradiated line beam (a sheet beam) along the line width direction andan energy level. FIG. 5 is an example where a beam profile is shaped ina trapezoid in which a level at the front area with respect to the scandirection is different from the rear area. FIG. 6 is an example where anedge of a beam profile shaped in a trapezoid is shaded off. FIGS. 7 and8 are examples of cases where that beam profiles are not as clearlydefined as in FIGS. 5 and 6. In all of the examples, the energy levelexceeds the threshold value energy, Eth, over which a silicon grain ismicrocrystallized in a line beam area corresponding to the front areaalong a scan direction of the line beam, i.e., the front area along theline width direction. However, the energy profile has a shape that theenergy level does not exceed the threshold value energy Eth in thebackward area relative to the front area. Therefore, it is possible toAnneal an amorphous silicon in an energy range GR which is optimal forobtaining a sufficiently large grain size and exists just below thethreshold value energy Eth, which allows the laser anneal processing forcrystallization to be performed in the optimal conditions.

[0064]FIG. 9 shows a relation where crystallinity depends on a filmtemperature when a laser beam is irradiated. Temperature is measured bya well-known crystal lattice state optical observation method, such as amethod using a pyrometer, and relates to energy of the ELA. In thisfigure, a manner of crystallization of an a-Si layer as a starting filmis shown. According to this manner, the film temperature rises as timepasses after irradiation begins, and the film temperature falls afterirradiation stops. With regard to the curve “a” in the figure, themaximum attainment point of the film temperature does not exceed thepolycrystallizing temperature Tg, therefore, even after the irradiationends, the film temperature being lowered and crystallization beinginactive, the state of the a-Si layer is maintained as it is. Withregard to the curve “b”, the maximum attainment point of the filmtemperature exceeds the polycrystallizing temperature Tg, therefore,after irradiation ends, the film temperature is lowered andcrystallization is inactive, and a p-Si(S) film with small size grainsis formed. With regard to the curve “c”, the maximum attainment point ofthe film temperature is higher than the curves “a” and “b”, therefore, ap-Si(L) film with large size grains is formed. With regard to the curve“d”, the film temperature exceeds the polycrystallizing temperature Tgand the microcrystallizing temperature TM over which a grain sizebecomes small, therefore, after irradiation ends, film temperature islowered and crystallization becomes inactive, causing a microcrystallinesilicon (M-cry) film to form. The difference of “a”, “b”, “c”, and “d”is caused by ELA energy. Accordingly, it is understood that the filmtemperature should be raised as high as possible such that thetemperature does not exceed the microcrystallizing temperature TM, forobtaining a p-Si(L) film having large grain size and satisfactoryannealing.

[0065]FIG. 10 shows a relation between irradiation time, specificallythe number of irradiated laser pulse shots, and grain size. The grainsize increases as the number of pulse shots and an ELA energy increase,in a condition that the film temperature is equal to or less than themicrocrystallizing temperature TM shown in FIG. 9. The grain sizerapidly increases over the initial several shots, and gently increasesby the succeeding shots.

[0066] This ELA property was derived from experimental results asdescribed below.

[0067] A p-Si layer formed by the ELA with relatively high energy, aswill be described in detail later, was first Secco etched. Its filmstate was then examined using an SEM, an optical microscope, and othermeans. As a result, an ELA property as shown in FIG. 11 was found. Theupper section of the figure is a beam profile showing a relation betweenlaser irradiation position and energy, wherein the horizontal axis[Position] refers to a position along the width direction of a line beamor a scan direction of the beam, and the vertical axis [Temperature]refers to film temperature, having a relation to the laser energy,measured by optical observation of a lattice state. The lower section ofthe figure is a view showing a layer state, processed by the ELA,corresponding to the beam profile. The beam profile is shaped in atrapezoid in which the temperature level at the top of the profileexceeds the microcrystallizing temperature TM, therefore a filmcorresponding to the top is in a microcrystal (M-cry) state. At thesides of the trapezoidal profile having a steep slope with a straightline or a smooth curved line, the film temperature rises and exceeds thepolycrystallizing temperature Tg and the microcrystallizing temperatureTM, as nearer the central part of the region to be irradiated by thelaser beam along the width direction, the top of the trapezoidalprofile, or the microcrystal region. The most outer part of the filmcorresponding to the temperature region equal to or less than thepolycrystallizing temperature Tg of the beam profile is in an a-Sistate, and the film corresponding to the temperature region from thepolycrystallizing temperature Tg to the microcrystallizing temperatureTM is in a p-Si state. A grain size of p-Si successively increased fromthe a-Si region, as the film temperature increased over thepolycrystallizing temperature Tg shown in the profile. The mostcharacteristic point was that the grain size increased according totemperature until temperature attained the microcrystallizingtemperature TM, and then grain size rapidly decreased to be changed intomicrocrystal over the microcrystallizing temperature TM. As a result,the maximum grain size was obtained at the highest temperature region MXwhich does not exceed the microcrystallizing temperature TM.

[0068]FIG. 12 is a view explaining a state of a processed film obtainedby scanning a line beam having an energy profile as shown in FIG. 11 onthe a-Si layer. The upper section of FIG. 12 is an energy profile of alaser beam identical to FIG. 11, the middle section of FIG. 12 is aschematic view of a state of the processed film obtained by laserannealling, and the lower section of FIG. 12 shows a relationship,connected with the middle section, between a film position correspondingto the processed film in the middle section along beam scan directionand grain size. FIG. 12 shows that an irradiated position is at theposition shown in the figure as the beam proceeded while the irradiatedposition was sequentially shifted from the left side of FIG. 12. In thisexperiment, a scan of the line beam was set such that each laser pulse95% overlapped each next pulse and twenty laser pulses were repeatedlyscanned for a unit irradiated region in one instance. FIG. 13A is anoptical microphotograph (magnification: ×125) of the processed filmwhich is a base of the schematic view of a state of the film shown inthe middle section of FIG. 12., FIG. 13B is an optical microphotograph(magnification: ×250) enlarging the left side region of FIG. 13A. FIG.13C is an optical microphotograph (magnification: ×250) enlarging theright side region of FIG. 13A.

[0069] In each laser pulse, p-Si of a sufficiently large grain size GMwas sequentially formed at a film corresponding to the MX region that isan edge part of the beam profile, while M-cry was formed at atemperature region over the temperature TM at the front area of the MXregion of the energy profile with respect to the scan direction, therebyforming a striped pattern. In other words, it is understood that whileM-cry formed by previous pulses is crystallized by succeeding pulses,fine linear p-Si is formed in the film region corresponding to thetemperature region MX for obtaining the maximum grain size GM while inother regions, an energy level by which a grain size of M-cry can bemore increased is not given. There is also an MX region for obtaining asufficiently large grain size at a front edge part of the beam profilealong the scan direction. However, M-cry is formed while the top of thebeam profile thereafter passes through.

[0070] A p-Si layer formed by relatively low energy ELA, in which thepeak energy level of a line beam was set such that it was lower than thebeam illustrated in FIG. 11, was Secco etched and its film state wasexamined using an SEM, an optical microscope, or the like, with a resultas shown in FIG. 14. The upper section of the figure is, as was the casein FIG. 11, an energy profile against an irradiation position of a laserbeam along a scan direction, and the lower section of the figure is aview shoving a state of a film processed by ELA obtained by irradiatinga laser beam with this beam profile. While the beam profile is trapezoidshaped as in FIG. 11, the energy level of the line beam is setrelatively low, therefore its top did not exceed the microcrystallizingtemperature TM, which allowed ELA to be performed at a temperatureregion over the polycrystallizing temperature Tg. As a result, p-Sihaving a not very large grain size was formed. The state of a-Si wasmaintained in its prior state, under the polycrystallizing temperatureTg.

[0071]FIG. 15 is a view explaining a state of a processed film obtainedby scanning the line beam having the energy profile shown in FIG. 14 onan a-Si layer. The upper section of FIG. 15 is an energy profile of alaser beam identical to FIG. 14, the middle section of FIG. 15 is aschematic view of a state of the processed film obtained by laserannealling, and the lower section of FIG. 15 shows a relation,corresponding to the number of laser shots, between a film positioncorresponding to the processed film in the middle section with respectto a scan direction of the beam and a grain size. FIG. 15 shows that anirradiated position is at the position shown in the figure at a time, ina condition that the line beam proceeds while the irradiated position issequentially shifted from the left side of FIG. 15. In this experiment,a scan of the line beam was set such that each laser pulse 95%overlapped each other pulse and twenty total pulses per scan wererepeatedly performed for a unit irradiated region. FIG. 16A is anoptical microphotograph (magnification: ×125) of the processed filmwhich is a base of the schematic view of a state of the film shown inthe middle section of FIG. 15. FIG. 16B is an optical microphotograph(magnification: ×250) enlarging the left side region of FIG. 16A. FIG.16C is an optical microphotograph (magnification: ×25.0) enlarging theright side region of PIG. 16A. At the conditions shown in FIGS. 15 and16A to 16C, microcrystal was not formed by change into an amorphousstate, because laser energy did not exceed the microcrystallizingtemperature TM, which allowed crystallization to proceed as the numberof pulses increased, thus enlarging the grain size.

[0072] An a-Si layer was crystallized by the initial four pulses tochange the grain size into Gg and the grain size further gentlyincreases as the number of pulses is increased. Finally, the grain sizeattained to the maximum size Gp obtained by ELA energy in thisexperiment at the 12th pulse. A large change in the grain size was notobserved thereafter.

[0073] The following information was revealed by the above describedexperiments. In order to increase silicon grain size by ELA, an increaseof an energy level was required. However, if an energy value exceeded acertain value, the silicon returned to an amorphous state and grain sizerapidly decreased, thus forming microcrystal. Referring to FIG. 17,which shows a relationship between ELA energy and grain size, the grainsize increased as energy increased. The grain size attained its maximumwhen energy exceeded Ed at which a sufficiently large grain size GMcould be obtained. Further increases in energy decreased, and soonrapidly decreased, the grain size. While it was not proven that theenergy level which maximizes the grain size strictly agrees with suchthreshold value energy, the energy level which maximized the grain sizein the experiments was very close to the threshold value energy and thethreshold value energy was larger than the energy level which maximizesthe grain size. Therefore, hereinafter, the energy level which maximizethe grain size and the threshold value energy over which the grain sizerapidly decreases are regarded to as identical for practical purposes.In other words, it is defined that such threshold value energy is aborder; the maximum grain size can be obtained just under the border andmicrocrystal is obtained over the border. Such threshold value energy isreferred to as the upper limit Eu of the allowed area.

[0074] It is understood from FIG. 17 that an energy level should be in arange from Ed to Eu to obtain a grain size over GM. Particularly, it isunderstood from the shape of the characteristic curve in the figure thatthe laser energy should be as high as possible within a range under thethreshold value energy over which silicon is changed into an amorphousstate to obtain maximum grain size. However, as shown in FIG. 3,unevenness of irradiated laser energy cannot be avoided. Therefore, whenan irradiated energy exceeds, even slightly, the threshold value energyover which the grain size rapidly decreases the corresponding regionbecomes a defective crystallization region, thereby degradingcharacteristics of a TFT formed in this region.

[0075] Accordingly, excellent ELA can be performed by an energy profilesuch that, as shown in FIGS. 5 to 8, the energy level gently decreasesfrom the front side along the scan direction of an irradiated laser beamto the backside to intersect the level of the threshold value energy Ethover which grain size rapidly decreases. The energy level of the beamprofile exceeds the threshold value energy Eth at the front area alongthe scan direction of the beam. The level lowers as nearer the rear areawith respect to the scan direction, allowing an annealing with themaximum energy which does not exceed the threshold value energy Ethwithin the region GR just below the intersection of the energy Eth andthe profile to be performed, thereby forming a p-Si layer having themaximum grain size. In other words, there is an energy region GR, whichmaximizes grain size, at just below the threshold value energy Eth overwhich a grain size rapidly decreases. Microcrystal is formed in a regionhaving an energy level over the threshold value energy Eth at the frontarea of the region GR. Passing through the region GR next forms a p-Silayer of maximum grain size. In the region at the rear of the region GR,the grains, once formed, are never microcrystallized, since the energylevel in this region does not exceed the threshold value energy Eth.Accordingly, it is possible to form a p-Si layer of maximum grain sizebecause of excellent annealing over the entire region of a film to beprocessed by setting an overlap amount and a pulse frequency such thatan area in the film to be processed is shot at predetermined timeswithin the region GR.

[0076] The structure of the above laser irradiation apparatus forperforming laser annealing will next be explained with reference to FIG.18.

[0077] In this figure, reference 51 denotes a laser oscillation source.References 52 and 61 denote mirrors. References 53, 54, 55, and 56denote cylindrical lenses. References 57, 58, 59, 62, and 63 denoteconvergence lenses. A reference 60 denotes a slit determining the linewidth direction, and a reference 65 denotes a slit determining the linelength direction. A reference 64 denotes a stage for supporting asubstrate 7 to be processed which has a surface where an a-Si layer isformed. The slit 65 is provided close to the stage 64.

[0078] Laser light is supplied by an excimer laser in a pulse wave form.Laser light irradiated from the laser oscillation source 51 is shaped bytwo pairs of condenser lenses consisting of a pair of cylindrical lenses53 and 55 and a pair of cylindrical lenses 54 and 56, into parallellight whose intensity has an almost flat output distribution in thelongitudinal and lateral directions. This parallel light is referring toFIG. 19, converged in one direction by lenses 58, 59, 62, and 63, and isreferring to FIG. 20, expanded in another direction by a lens 57, tocreate a band-like, rectangular, or linear for practical purposes, beamthat is irradiated on the substrate 7 to be processed. A slit 60 foradjusting the line width and a slit 65 for adjusting the line lengthrespectively shield both end portions in the line width direction [A],and in the line length direction, to clearly define the shape to beirradiated, thus generating a line beam of a width A while constantlymaintaining the intensity in the effective irradiation region.

[0079] The stage 64 where the substrate 7 is mounted is arranged to bemovable in the X- and Y-directions. With use of this kind of apparatus,the annealing processing as described above can be carried out with ahigh throughput for a substrate of a large area, even when annealingprocessing is carried out in a plurality of steps.

[0080] In addition, the laser irradiation apparatus as described aboveis capable of arbitrarily setting a distance between the focal point ofthe laser beam and the substrate 7 to be processed by the optical systemshown in FIGS. 18 to 20 and adjusting an energy profile of the laserbeam irradiated on the substrate 7 by setting the distance at apredetermined value as described below.

EXAMPLES

[0081]FIG. 21 is a graph of a first practical example, showing a beamprofile with regard to the ELA apparatus shown in FIG. 18 in a case inwhich a distance between a focal point of a laser beam and a substrateto be processed was set at 300 μm. In this graph, the energy level had ashape that spikes to the left side of the profile. FIGS. 22A, 22B, and22C are optical microphotographs showing when scanning was performed insuch a manner that a scan direction of a line beam with the energyprofile shown in FIG. 21 was set in the left direction in FIG. 21. It ispossible to examine a state of the film by observing interfered light,caused by a difference of a grain size, which had different colordepending on grain size because of Secco etching performed. FIGS. 22A,22B, and 22C correspond to energy densities of 390 mJ/cm², 400 mJ/cm²,and 410 mJ/cm² respectively. FIGS. 23A, 23B, and 23C are opticalmicrophotographs taken when scanning is performed in such a manner thata scan direction of a line beam with the energy profile shown in FIG. 21is set in the right direction. In FIGS. 22A, 22B, and 22C, there arevery few black points in which defective crystallization regions existand those that are present are not localized, showing a p-Si layerhaving excellent quality is formed on the other hand, in FIGS. 23A, 23B,and 23C, vertically linear defective crystallization regions shown inblack are recognized. Further, as shown in FIG. 23C, linear blackregions are quite prominent, showing the existence of a large area ofdefective crystallization, and that the film is of a remarkably lowquality.

[0082] The following can be inferred from the above observations. In theline beam having the energy profile shown in FIG. 21, the jumping partis subject to exceed the microcrystallizing temperature or the thresholdvalue energy Eth over which a grain size rapidly decreases. Especially,the more enhanced the laser energy is, the higher the probability thatsuch event will occur. When the line beam was scanned in the rightdirection of FIG. 21, the film to be processed was annealed while thevery end of irradiated region along the scan direction ismicrocrystallized. Accordingly, p-Si formed at the front area withrespect to the scan direction was changed into microcrystal at the rearof the beam and remains as defective crystallization regions as shown inFIGS. 23A, 23B, and 23C. Such events are apt to occur with higherenergies. Therefore, in attempts to obtain a large grain size, a problemoccurs that microcrystal is formed.

[0083] On the contrary, when the line beam was scanned in the leftdirection of FIG. 21, microcrystal tended to form toward the front areaof the beam. Thereafter, however, excellent crystallization wasperformed within the energy region just below the threshold value energyas the beam passes, thereby forming p-Si having a large grain size. InFIG. 22C, few defective crystallization regions in black are recognized,showing that the upper limit of the energy range to be set is less than410 mJ/cm².

[0084] Therefore, it is understood with the beam line having the energyprofile as shown in FIG. 21 that an ideal annealing for crystallizing isperformed in conditions that the energy of the beam is set at about 400to 410 mJ/cm² and a scan direction is in the left direction of thefigure. In these conditions, microcrystal is formed at the front area ofthe beam. However, there is an area between the center area and the reararea of the beam that the energy level shifts from an energy regionhigher than the threshold value energy Eth over which a grain sizerapidly decreases to an energy region lower than the threshold valueenergy Eth, thereby existing an optimal beam region GR in which an idealannealing is performed. In this example, the laser beam is scanned suchthat a total of twenty pulses are performed. Thus, the number of pulseswithin the optimal beam region GR is less than twenty. However, as shownin FIGS. 9, 10, and 15, the initial several pulses complete formation ofgrains. Accordingly, it is possible to perform an excellent annealing bysetting an overlap amount and a pulse frequency to optimal values suchthat an area in the film to be processed receives beam pulses atpredetermined times.

[0085]FIG. 24 is a graph, as a second example, showing a beam profilewith regard to the ELA apparatus shown in FIG. 18 in a case where thedistance between a focal point of a laser beam and a substrate to beprocessed was set at 600 μm. Change of the focal distance in this mannerallows deformation of the beam profile utilizing an infinitesimal gap inlight caused by diffraction and interference. FIGS. 25A, 25B, and 25Care optical microphotographs taken when scanning was performed in such amanner that a scan direction of a line beam with the energy profileshown in FIG. 24 was set in the left direction. FIGS. 26A, 26B, and 26Care optical microphotographs from th right direction. FIGS. 25A, 25B,and 25C, and, FIGS. 26A, 26B, and 26C correspond to energy densities of390 mJ/cm², 400 mJ/cm², and 410 mJ/cm², respectively. The crystallinestates shown in FIGS. 25A, 25B, 26A, and 26B, are excellent, while,those in FIGS. 25C and 26C, show a greater amount of defectivecrystallization regions. Especially, in FIG. 26C, defectivecrystallization regions formed are outstanding. It is inferred from thisfact that the upper limit of the energy range to be set is about 410mJ/cm². In the laser beam profile of this example, as shown in FIG. 24,the profile has a shape that it is relatively high in the right side ofthe profile. Accordingly, when scanning was performed in the leftdirection, microcrystal was formed toward the rear scanning region ofthe laser beam, which shortened the period of annealling within theoptimal beam region GR in which an excellent annealing is performed tojust below the threshold value energy Eth, past which grain size rapidlydecreases. In other words, an annealing period under such optimalcondition is shortened. Therefore, crystallization of microcrystal wasnot sufficient, which in turn led to the formation of defectivecrystallization regions as shown in FIG. 25C.

[0086] On the other hand, when the line beam was scanned in the rightdirection, in the initial stage of the laser energy irradiation, anannealing with an energy level over the threshold value energy Eth wasperformed. Thereafter, as in FIG. 7, since the period to be annealedwithin the optimal beam region GR in which an excellent annealing isperformed just below the threshold value energy Eth is sufficientlylong, a p-Si layer with an excellent crystallinity, as shown in FIGS.26A to 26C, was obtained.

[0087] A third example follows. In this example, a distance between afocal point of a laser beam and a substrate to be processed was set at900 μm. FIG. 27 is a graph showing a beam profile in this case. FIGS.28A, 28B, 28C, 29A, 29B, and 29C are optical microphotographs taken whenscanning was performed in such a manner that a scan direction of a linebeam with the energy profile shown in FIG. 27 was set in the left andthe right directions, respectively. FIGS. 28A, 28B, and 28C, and FIGS.29A, 29B, and 29C correspond to energy densities of 390 mJ/cm², 400mJ/cm², and 410 mJ/cm² respectively, as was the case for the previousexamples. In all of these FIGS. 28A to 29C, the crystalline state isexcellent. In the laser beam profile shown in FIG. 27, the profile has ashape, similar to FIG. 8, where a portion of the graph peaks in energyto the right of the center area of the profile, and the energy levelgently decreases at both sides of the peak. Accordingly, regardless ofthe scan direction, which may be either the left direction or the rightdirection, the optimal beam region GR, in which a sufficiently largegrain size can be obtained, exists at an energy level shifting from aregion higher than the threshold value energy Eth to a region lower thanthe threshold value energy Eth, thereby enabling excellent annealing tobe performed.

[0088] Finally, a construction example in which a transistor device,specifically a thin film transistor, was formed utilizing a polycrystalsilicon film obtained by the above-described laser annealing method willbe explained.

[0089]FIG. 30 is a plane view of a TFT formed on the substrate 7 to beprocessed shown in FIG. 4 according to the present invention. A p-Silayer formed by the ELA method according to the present invention wasetched into an island-like shape for use in a TFT. In the p-Si 11 thusformed, a non-doped channel region Ch, light-doped regions LD, andheavy-doped source and drain regions S and D were formed. On the channelregion CH, a gate electrode 13 was formed with a gate insulating filminserted between the channel region and the gate electrode 13.

[0090]FIG. 31 shows an example of a cross-section where a TFT iscompleted. A p-Si layer 31 is formed in an island-like shape on anon-alkaline glass substrate 7 as a substrate to be processed, anon-doped channel region CH is formed in the p-Si layer 31, and regionsLD are formed in both sides of a non-doped channel region CH. Source anddrain regions S and D are formed outside the regions LD. A gateinsulating film 12 covers the p-Si layer 31, and a gate electrode 13consisting of a doped p-Si layer 13 p, tungsten silicide 13 s, or thelike is formed at a region corresponding to the channel region CH. Animplantation stopper 14 for preventing counter-doping when implantationions of a different conductive type in the CMOS structure is formed onthe gate electrode 13. Side walls 15 of the gate electrode 13 are formedto previously prepare margins so that the regions LD might not beenhanced over the edges of the gate electrode 13 when impuritiesimplanted into the p-Si layer 31 are diffused in the lateral directionby activation annealing. A first inter-layer insulating film 16 isformed so as to cover the entire surface of the substrate 7, and drainand source electrodes 17 and 18 made of low-resistance metal are formedon the first inter-layer insulating film 16 and are respectivelyconnected with drain and source regions D and S through contact holes CTformed in the gate insulating film 12 and the inter-layer insulatingfilm 16.

[0091] If a TFT as shown in FIGS. 30 and 31 is used and constructed in aCMOS structure which is used as a driver circuit section (including agate driver 23 and a drain driver 24) for an LCD as shown in FIG. 4, itis possible to simultaneously form a driver circuit with highperformance and high speed in manufacturing steps substantially equal tothose for a TFT for driving liquid crystal.

[0092] Further, in the display area 22 of the LCD shown in FIG. 4, asecond inter-layer insulating film 19 having a flattening effect isformed on the entire surface so as to cover the drain electrode 17 andthe source electrode 18 formed as shown in FIG. 32. In addition, a pixelelectrode for driving liquid crystal is formed on the second inter-layerinsulating film 19 and is connected with the source electrode 18.

[0093] For forming a liquid crystal display, another substrate isprovided so as to face the substrate 7 on which TFTs and pixelelectrodes connected thereto are formed. A liquid crystal layer isformed between the substrate. A common electrode coupled with a pixelelectrode to constitute a liquid crystal drive capacitor is formed onthe substrate facing the substrate 7.

[0094] As has been described above, by forming a polycrystal siliconfilm having a large grain size by an suitable laser annealing andutilizing the polycrystal silicon in a thin film silicon used forvarious devices (for example, a liquid crystal display), it is possibleto form a transistor with a excellent operating characteristics by a lowtemperature process.

[0095] While there have been described what are at present considered tobe preferred embodiments of the invention, it will be understood thatvarious modifications may be made thereto, and it is intended that theappended claims cover all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:
 1. A semiconductor layer laser annealling method forimproving characteristic of a semiconductor layer formed on a substrateby irradiating a laser beam, wherein an energy level in a region to beirradiated by the laser beam is set such that a level towards the rearof a region along which the laser beam scans is lower than that at thefront area or the center area of the region.
 2. A semiconductor layerlaser annealling method for obtaining a polycrystal semiconductor layerby irradiating a laser beam on an amorphous semiconductor layer formedon a substrate, wherein an energy level in a region to be irradiated bythe laser beam is set such that a level towards the rear of a regionalong which the laser beam scans is lower than that at the front area orthe center area of the region.
 3. A laser annealling method according toclaim 2, wherein the energy level at the front or center of the regionis equal to or greater than the upper limit energy level, which therebymaximizes grain size of the semiconductor layer.
 4. A semiconductorlayer laser annealling method for obtaining a polycrystal semiconductorlayer by irradiation of an amorphous semiconductor layer formed on asubstrate with a laser beam, wherein an energy level in a region to beirradiated by the laser beam is set such that the peak level in the reararea of a region along a scan direction of the laser beam is lower thanthe upper limit energy level which maximizes semiconductor layer grainsize.
 5. A laser annealling method according to claim 4, wherein thepeak level of the laser beam at the front area or the center area of theregion along the scan direction of the laser beam is equal to or greaterthan the upper limit energy level which maximizes a grain size of thesemiconductor layer.
 6. A laser annealling method according to claim 4,wherein the laser beam irradiated on the amorphous semiconductor layeris obtained by shaping a laser beam generated from a laser oscillationsource by an optical system including a plurality of lenses, such thatthe region to be irradiated has a predetermined shape, and the energylevel, energy distribution, or their combination in the region to beirradiated by the laser beam are controlled by adjusting a distancebetween the amorphous semiconductor layer formed on the substrate andthe focal point of the laser beam formed by the optical system.
 7. Alaser annealling method according to claim 6, wherein the scan directionof the laser beam is set such that an energy level at the rear area ofthe region along the scan direction is lower than the upper limit energylevel.
 8. A laser annealling method according to claim 4, wherein theupper limit energy level which maximizes a grain size of thesemiconductor layer corresponds to the lower limit energy level overwhich the polycrystal semiconductor layer is changed into an amorphousstate.
 9. A transistor device in which a polycrystal semiconductor layeris formed by subjecting an amorphous semiconductor layer formed on asubstrate to laser anneal processing, wherein an energy level in aregion to be irradiated by a laser beam of the amorphous semiconductorlayer is set such that the level in a rear area of a region along a scandirection of the laser beam is lower than the upper limit energy levelwhich maximizes a grain size of the semiconductor layer, and theamorphous semiconductor layer is annealed by the laser beam and thepolycrystal semiconductor layer obtained is used as an active layer ofthe transistor device.
 10. A transistor device according to claim 9,wherein the transistor device is a thin film transistor, and a channellayer of the thin film transistor is formed in the polycrystalsemiconductor layer obtained by the laser anneal processing.
 11. Atransistor device according to claim 9, wherein the transistor device isa thin film transistor, a channel layer of the thin film transistor isformed in the polycrystal semiconductor layer obtained by the laseranneal processing, and the thin film transistor is used as a switchingdevice formed in a display area of a substrate forming a liquid crystaldisplay and as a switching device of a driver circuit formed surroundingthe display area of the substrate through a process substantially equalto a process of forming the switching device of the display region.